In recent years, in the field of radio communication, a MIMO (Multiple-Input and Multiple-Output) transmission scheme has been utilized, which realizes an improvement in the speed and quality of signal transmission by executing transmission and reception using multiple antennas on each of the transmitter side and the receiver side.
Moreover, in order to further improve signal transmission speed and further reduce interference, a massive MIMO transmission scheme has been considered, which uses a mass of antenna elements (e.g., 100 elements or more) in a high frequency band (e.g., 10 GHz or more) that enables antenna miniaturization and in which a wide bandwidth can be secured (e.g., Patent Document 1).
In massive MIMO, advanced beamforming (BF) is executed that uses a larger number of antenna elements in comparison to conventional MIMO. Beamforming is a technology that involves controlling of characteristics, such as the directivity, shape, or gain, of a beam (e.g., a transmission beam corresponding to a transmitting antenna, a reception beam corresponding to a receiving antenna) by controlling antenna elements. With MIMO, the phase and the amplitude can be controlled for each antenna element, and therefore, the flexibility of beam control increases with an increase in the number of antenna elements that are used.